Galvanic cells, such as alkaline cells, are generally designed to vent when the internal pressure exceeds a predetermined amount. When exposed to an abuse condition, such as being charged to an excessive degree, the cell is designed to vent and allow gas to escape. Under certain abuse conditions, electrolyte entrained in the gas may be forced from the cell. It is preferable to have the electrolyte escape rather than have the cell rupture from internal pressure buildup.
Cell manufacturers have used a number of approaches to resolve the problem of expelling electrolyte during venting. One method of preventing seal rupture due to abuse charging or the like is to insert a diode in the battery's electrical circuit. By eliminating the possibility of charging the cells, internal gas is not generated and the seal never ruptures. Another electrically related mechanism is a belleville shaped "flip switch". This device is triggered by bulging of the closed end of the cell's cylindrical container which causes a washer to invert and thereby break electrical contact. Another method involves the use of absorbents or electrolyte thickeners. The absorbent materials are usually located outside the seal area and beneath the cell's cover or jacket. As electrolyte escapes from a ruptured seal, the liquid is absorbed. Spew thickeners are mixed with the electrolyte and therefore are contained within the cell. The objective of the thickener is to slow down and/or absorb any leakage that may occur. The disadvantage of using either an absorbent or a thickener is that both materials tie up space that otherwise could be used for active materials of the cell. A third procedure is to use an outer container and end covers as an electrolyte containment system to provide space to contain the electrolyte that may escape.
U.S. Pat. No. 3,676,221 discloses a battery comprising a plurality of stacked, disk-like sealed cells secured together by cups fitted over one cell and having bottoms spot-welded to the next cell and sidewalls spot-welded to the interfitting cell. A heat-shrunk sheath encloses the battery and has caps forming the poles. Between each pair of cells is a circular disc of insulating material against which the cup bottoms bulge upon expansion of the contents of the cells, thereby breaking the welds and electrically disconnecting the cells.
U.S. Pat. No. 4,025,696 describes a disk shaped washer which inverts after the bottom bulge exceeds a predetermined value. Prior to activation, the washer's inside diameter is slanted toward the container. As the container bulges, the bottom of the container pushes against the washer and eventually causes the washer to invert. This inversion electrically separates the bottom cover from the container. An open circuit is the net result.
U.S. Pat. No. 3,775,661 describes a cell in which internal pressure forces a diaphragm against a switch which electrically disconnects a charging device. The diaphragm is located inside a venting device which is attached to one end of the cell.
U.S. Pat. No. 2,651,669 describes a bulge activated switch that can be incorporated into a single cell battery or a multiple cell battery and operable such that the bulge can be used to open a switch or switches that control the cell's discharging and/or charging circuits.
U.S. Pat. No. 3,617,386 describes a cell in which a thin sheet of metal with "spring back" ability is positioned between the seal and cover of the cell so as to break the cell's electrical circuit when the bulge becomes excessive.
U.S. Pat. No. 3,081,366 describes a sealed cell having a metallic sheet member connected to one cell electrode and its periphery insulating affixed to an open casing end and an overlying exposed metallic terminal insulating held over the sheet member. A movable switch portion normally connects an intermediate pressure-deflectable sheet member portion to the external terminal and, in response to outward motion of the deflected sheet portion under excess internal pressure the switch portion disconnects the external terminal from the deflected sheet portion.
U.S. Pat. No. 3,373,057 describes a cell in which the cover of the casing of the cell is provided centrally with an inwardly concave-contact button. A dished (which is to say concave-convex) snap-acting spring disc of the automatic reset type is marginally sealed to the inside of the cover. An automatic reset disc after snapping in one direction in response to pressure on its convex side will return with snap action when the pressure is relieved. The disc is provided centrally with a sealed movable contact for engagement and disengagement with an internal fixed contact when the disc snaps to and fro. The arrangement is such that when the contacts are engaged the disc is slightly sprung toward the cover but short of causing snap action. This maintains good electrical contact pressure under safe internal gas pressures. The fixed contact is electrically connected with one set of battery plates and the other set of plates is electrically connected with the casing.
U.S. Pat. No. 4,690,879 describes a cylindrical galvanic cell employing a unitary type cover welded to the container as a failsafe circuit interruption means for electrically isolating one terminal of the cell from the cell's electrochemical system when the bottom of the cylindrical cell bulges beyond a predetermined amount so that the unitary cover breaks electrical contact with the housing of the cell.
U.S. Pat. No. 4,756,983 describes a cylindrical galvanic cell employing a cover having a peripheral flange electrically contacting the conductive container of the cell and whereby a predetermined bulge in the bottom of the container will cause the central portion of the cover to separate from the peripheral flange portion thereby electrically isolating the central portion of the cover from the cell.
It is therefore an object of this invention to provide a galvanic cell with a more accurate means for electrically isolating one terminal of the cell from the cell's electrochemical system upon reaching a predetermined internal pressure in the cell's housing.
Another object of this invention is to provide means for electrically isolating one terminal of a galvanic cell from the cell's electrochemical system upon reaching a predetermined bulge in the cell's housing and that will occupy minimum additional space so as not to significantly diminish the space allocated for the active components of the cell.
Another object of this invention is to provide a sealed galvanic cell with means for electrically isolating one terminal of the cell from the electrochemical system of the cell upon reaching a predetermined bulge in the cell's housing and that is easy to make, cost effective and easy to assemble.
The foregoing and additional objects will become more fully apparent from the following description and the accompanying drawing.